This invention relates to thermal insulation materials which contain expanded perlite and alkali metal silicate as the principal constituents.
The use of expanded perlite bonded with alkali metal silicates for thermal insulation is described in U.S. Pat. Nos. 3,658,564 and 4,288,253. These patents disclose a process for producing water-insensitive bonded perlite structures using fillers having at least 75% by weight of expanded perlite and an aqueous solution (28 to 34% solids) of sodium silicate having a weight ratio of SiO.sub.2 to Na.sub.2 O of between 3:1 and 4:1 or a potassium silicate solution (24 to 30% solids) having a weight ratio of SiO.sub.2 to K.sub.2 O of between 2:1 and 2.6:1. In the '564 patent the mixture of expanded perlite and alkali metal silicate is molded by compression and then cured preferably at a temperature of about 275.degree. to 400.degree. F. under controlled humidity conditions (about 50% relative humidity) with the water content of the molded article being maintained at 10% by weight. An extended curing and drying period of at least three days, and preferably seven days, is used to achieve water insensitivity. According to the '253 patent, the compositions of the '564 patent can be improved by use of a temperature of about 180.degree. to 225.degree. F. and a relative humidity of about 30% to 60% for the curing step.
Unfortunately, such prior art expanded perlite/alkali metal silicate insulation material have serious deficiencies and fail to meet some important industrial requirements. The most important criteria of thermal insulation materials are low density, low thermal conductivity, high strength, good impact resistance, low water absorption or good water repellency, and stability to high temperature exposure. To be suitable for use at high temperatures, it is essential that the material be adequately stable at high temperatures without excessive shrinkage, warpage or cracking. An insulation material designed for use up to 1200.degree. F. should have an average maximum linear shrinkage as tested per American Society for Testing Materials (ASTM) standard # C-356 of 2.0% or less at temperatures up to 1200.degree. F. Expanded perlite/alkali metal silicate insulation material made in accordance with the teachings of the '564 and the '253 patents do not meet this standard. For example, a pipe insulation having a composition identical to the only example of the '253 patent showed an excessive linear firing shrinkage of about 9.0% or higher when fired at 1200.degree. F. for 24 hours.
Additionally, the requirement that the expanded perlite/alkali metal silicate insulation material be cured under closely controlled temperature and humidity conditions, as taught by both the '564 and the '253 patent poses problems in the large-scale manufacture of such materials. Moreover, such parameters make commercial production very costly.